1. Field of the Invention
The present invention relates to a graphene screening and separation method, and more particularly to a graphene screening and separation device.
2. Related Art
Graphene is a single planar sheet of sp2-bonded carbon atoms, that are packed in a honeycomb crystal lattice. The multi-layer graphene is a layer by layer three-dimensional structure.
Both mechanical strength and flexibility of the graphene are better than those of the transparent indium tin oxide (ITO) material, and thus the transparent conductive electrode can replace the ITO with the graphene. The transparent conductive electrode requires 3 to 5 layers of graphene for concerning transmission coefficient and surface resistance.
The charging interval of super-capacitors made of graphene only requires 1 millisecond. The specific surface area of single-layer graphene can reach 2630 m2/g, which is a very good energy storage material for the super-capacitor. In use, the ideal material for the super-capacitor is single layer graphene.
The graphene can replace silicon, and is applied to the field of transistors. In use, the material of transistor requires 1 to 2 layers of graphene, which can increase the transistor speed to the THz level.
The negative electrodes of lithium-ion batteries which are made by graphene can significantly improve the battery performance. In use, the battery negative electrode requires 3 to 4 layers of graphene with cavities there-between. The battery negative electrode using graphene material has higher output power density in a low temperature environment, and its performance is not easily deteriorated when charged and discharged repeatedly.
The above products require a particular number of graphene layers material, and in the current process technology, its purity has not yet been focused on the particular number of graphene layers. Therefore, the particular number of graphene layers must be separated with identification as well as separation and purification methods.
The conventional method for identifying the number of graphene layers is an optical method. There are other methods. For example, the number of graphene layers is identified with Quantum Hall effects, so as to distinguish single-layer graphene from double-layer graphene. The disadvantage is that the identification of the number of graphene layers with Quantum Hall effects needs to be carried out in a high magnetic field environment and thus is impractical and inefficient. Alternatively, the number of graphene layers is identified with Raman spectrum. For instance, scattered photons are measured after phonons and photons are interacted, and the thickness is identified according to Raman spectrum of graphene. The disadvantage is that differences between two layers and a few layers of graphene sheets are not obvious in Raman spectra.
Alternatively, the number of graphene layers is identified with contrast spectrum. For example, under irradiation of a white light source, the obtained reflection spectra were compared with that of a background spectrum from SiO2/Si to generate the contrast spectra. The disadvantage is that the contrast can be displayed only for a particular thickness and a substrate made of a particular material, which is difficult to be combined with the existing separation system.
The conventional method of separating and purifying graphene layers is density gradient ultracentrifugation. For instance, graphene suspension is formed by using amphiphilic surfactant sodium cholate, and the graphene layers are separated with density gradient ultracentrifugation. This method solves the problem of the coupling effect between area and thickness polydispersity in general gravity centrifugation. The disadvantage is that batch separation of different layers of graphene has a bottleneck in purity upgrade, and currently the single-layer graphene may reach about 85% by this method.
As stated above, the optical method is a relatively practical method of identifying the number of graphene layers currently. However, when the particular number of graphene layers is to be separated from the graphene suspension, the density gradient ultracentrifugation is mostly used. Although the optical method of identifying the number of graphene layers is clear, to separate a particular number of graphene layers is still from other method.
Therefore, there is a requirement to provide a screening and separation method and device for graphene to solve the problems.